Bioresorbable and/or biodegradable polymers (i.e. biopolymers) can be divided into natural and synthetic polymers. To the natural polymers belong e.g. proteins, polysaccharides and lignin. Synthetic biopolymers are e.g. aliphatic polyesters, polyorthoesters, some aliphatic polycarbonates, polyanhydrides and some polyurethanes. Biopolymers can also be produced by microbes e.g. polyhydroxy alkanoates. The most important group of biodegradable polymers is based on aliphatic polyesters, the degradation of which is mainly based on hydrolysable ester bonds. Bioresorbable polymers degrade in the physiological environment and the degradation products are eliminated through the kidneys or completely bioabsorbed. According to strict definition, biodegradable polymers require enzymes or micro-organisms for hydrolytic or oxidative degradation. But in general, a polymer that loses its mass over time in the living body is called an absorbable, resorbable, bioresorbable or biodegradable polymer. This terminology is applied in the present invention regardless of polymer degradation mode, in other words for both enzymatic and non-enzymatic degradation and/or erosion.
Biodegradable polymers are used and studied in an increasingly large number of biomedical applications, such as controlled drug delivery devices, implants and resorbable sutures, as well as mass produced applications such as packaging, paper coating, fibres, films and other disposable articles. These applications bring special requirements to the polymers and monomers. These polymers are generally required to be biodegradable and non-toxic, or in the biomedical applications, bioresorbable and/or biocompatible. On the other hand, polymers should have good chemical, mechanical, thermal and rheological properties.
In the last few decades, novel controlled drug delivery systems have attracted interest due to their potential advantages. For example, the safety and efficacy of many drugs can be improved if they are administered by novel delivery systems. For many drugs a constant plasma concentration is desirable, especially for those drugs exhibiting narrow therapeutic indexes. Bioabsorbable devices represent the state of the art in drug delivery and in managing orthopaedic problems such as the use of implants in fracture fixation and ligament repair. Biodegradable polymers applied as drug delivery systems generally require no follow-up surgical removal once the drug supply has been depleted. Mainly implantable rods, microspheres and pellets have been investigated.
Polycaprolactone (PCL) is among the most common and well-studied bioresorbable polymer. The repeating molecular structure of PCL homopolymer consists of five non-polar methylene groups and a single relatively polar ester group. This high molecular weight polyester is conventionally produced by the ring-opening polymerisation of the cyclic monomer, i.e. ε-caprolactone. A catalyst is used to start the polymerisation and an initiator, such as an alcohol, can be used to control the reaction rate and to adjust the average molecular weight. PCL is a semi-crystalline (˜40-50%), strong, ductile and hydrophobic polymer with excellent mechanical characteristics having a low melting point of 60° C. and a glass transition temperature of −60° C.
Poly(ethylene glycol) is a biocompatible and highly water soluble (hydrophilic) polymer. Poly(ethylene glycols) are low molecular weight (<20000 g/mol) poly(ethylene oxides) containing the repeat unit —CH2CH2O—. PEG is a highly crystalline (˜90-95%) polymer having a low melting point of 60° C. and a glass transition temperature of −55 to −70° C. These difunctional compounds contain hydroxyl end-groups, which can be further reacted and chain extended with diisocyanates or used as initiators for ring-opening polymerisations. PEGs are well-known structural units incorporated into crosslinked polyurethane hydrogels (EP publications EP0016652 and EP0016654) and linear polyurethane hydrogels (PCT publication WO2004029125).
Amphiphilic block copolymers, e.g. PEG-PCL copolymers, have recently attracted attention in the field of medicine and biology as micellar carriers, polymer vesicles and polymer matrices. The triblock copolymer PCL-PEG-PCL has unique phase behaviour in blends and the ability to form polymeric micelle-like core-shell nanostructures in a selective solvent, in which only one block is soluble (J. Polym. Sci. Part A Polym. Chem., 1997, 35, 709-714; Adv. Drug Delivery Rev., 2001, 53, 95-108).
However, the above-mentioned polymers suffer from a number of practical disadvantages. The degradation rate and mechanism appear to depend on a number of factors, such as the chemical structure of the polymer and on the surrounding environmental conditions, such as the degradation media. Two stages have been identified in the degradation process of aliphatic polyesters. Initially, the degradation proceeds by random hydrolytic chain scission of the ester bonds, leading to a decrease in the molecular weight; in the second stage measurable weight loss in addition to chain scission is observed. Another observation is that polycaprolactone degrades much slower than e.g. polylactide. The long degradation time of polycaprolactone (˜24 months) is usually a disadvantage for medical applications.
It is an object of the present invention to obviate and/or mitigate the disadvantages of the known bioresorbable polymers. In particular, it is an object of the present invention to provide a consistent and/or flexible approach to providing polymers having differing degradation properties which may be chosen according to the intended use of the polymers, including providing polymers having differing degradation rates. It is a further object to provide bioresorbable polyurethane polymers which fulfil one or more of these objects. A preferable object is to provide bioresorbable polyurethane polymers which are non-toxic on degradation.